The field of the invention is rotary printing press technology, and in particular high speed rotary presses for producing high volume output such as newspaper production and the like.
In all rotary printing press machines, a common problem is control of the tension of the paper web passing through the printing cylinder and the associated feed rollers, angle bars, folders, and the like. The paper web undergoes an increase in tension as it transits these parts of the printing press setup, and the increase in tension, termed gain, is a siginficant factor in press setup and operation.
Gain is due primarily to differences in rotational speeds of the rollers over which the paper web passes, resulting in surface speed differences which apply tension differentials to the paper web. When the web tension exceeds the tensile strength of the paper, the paper web breaks, causing press shutdown and requiring a press crew to reset the paper in the press and the roller setup. Any down-time of the printing press represents a significant loss of production capacity. In addition, the printing business is frequently dealing with deadlines, and the loss of revenue due to missing deadlines can be a crucial factor in printing press profitability.
It has been noted that frictional forces appear to comprise a more important component of gain in the Flexo printing process as compared to prior art letterpress operation. This fact is apparently due to the fact that the Flexo process commonly uses water-based ink, whereas other printing processes use oil-based ink. The oil-based inks produce a lower coefficient of friction as the web slides over the rollers and angle bars. In contrast, the water-based inks used in the Flexo process moisture is absorbed into the paper, creating a dry surface with a correspondingly higher coefficient of friction. This phenomenon in turn creates increased frictional forces at each contact surface between the paper web and the rollers and angle bars of the printing setup. These greater frictional forces must be overcome by increased pull from the drag roller and the RTF, causing higher web tension that must be compensated during press setup.
Several strategies are known in the prior art for dealing with this web tension problem. The individual leads from the printing press units may be overfed or underfed to compensate for the inherent differences in resultant web tension, which is a function of varying lead lengths and roller/angle bar configurations. Over- and underfeeding from printing press units is limited in the Flexo setup by the fact that a Flexo plate can be damaged by excessive impression, a result of overpacking. Considering a common base line of 0.145 inch range of overpacking on a 44 inch printing cylinder, it is possible to obtain only a 0.3% adjustment to the gain, far below what is required to compensate for many lead configurations.
Alternatively, it is possible to adjust the folder bands, nips, and RTF. However, adjusting the folder is limited by the fact that folder adjustment results in uniform changes to the pull on all the leads, which does not solve the problem of varying lead tension setup requirements. It is also possible to install Velcro on critical roller surfaces to reduce frictional effects of web travel. This is a labor intensive task which must be repeated frequently as a function of wear. Although Velcro reduces the coefficient of friction of the web, it falls far short of the slidability provided by oil-based inks.
An obvious answer to the problem of gain is a mechanical solution: alter the gear ratio of the drag rollers with respect to the printing press to select the optimum drag for what is deemed to be the most common lead configuration. If a fixed lead configuration is used, this approach will work. However, in most printing operations, such as newspaper production, the lead configuration is altered frequently in accordance with changes in the configuration of the printed output.